Millimeter-sized devices, which are implanted within tumors by a minimally invasive operation technique, are very promising for tumor treatment with a contactless thermal ablation procedure. These implanted devices are heated from outside of the patients' body by an alternating magnetic field. For minimizing unwanted influencing and heating of healthy tissue and other devices inside the patients' body, the ratio of generated heating power within the implanted devices to required magnetic field strength has to be maximized. In this paper, the heating power generated by eddy currents within solid and electrically conductive implanted devices, whose dimensions are restricted by the minimally invasive operation technique, is analyzed and optimized based on a combination of numerical and analytic calculations for different parameter sets of material and magnetic field properties. The parameters for achieving maximum heating power are presented along with the dependency of the heating power on these parameters. The results are validated by experimental prototype measurements. Furthermore, the specific absorption rate is evaluated based on specific coil configurations for generating the alternating magnetic field. This paper shows the feasibility of significantly increasing the heating power of solid and electrically conductive implanted devices by choosing the appropriate properties of the implant material. This enhances the safety and the well-being of the patients and represents a great benefit for the outcome of the tumor treatment.